This invention relates to a method of producing a stock material for cerium-based abrasives comprising cerium oxide as the main ingredient, and also to a cerium-based abrasive of excellent grinding characteristics, produced from the above stock material.
Cerium-based abrasives have been used for grinding various glass materials. Recently, in particular, their applicable areas have been expanding, as they are used for grinding glass materials for electric and electronic devices, e.g., glass as magnetic recording medium for hard disks or the like and glass substrates for liquid crystal displays.
A cerium-based abrasive is composed of cerium oxide (CeO2) particles as the main ingredient and abrasive particles of another rare-earth metal oxide. It is classified into two general categories, high-cerium and low-cerium, by a proportion of a cerium oxide content to a total rare-earth oxide (hereinafter referred to as TREO) content. The methods of producing these types are not much different. More concretely, the method starts with crushing the stock material followed by chemical treatment (wet treatment) for each type. The chemical treatment methods include fluorination, i.e., incorporation of a fluorine component to secure high grindability for the cerium-based abrasive, and treatment with a mineral acid to remove an alkaline metal, e.g., sodium, for prevention of abnormal growth of the particles during a roasting process. The wet-treated stock material is subjected to filtration, drying, roasting at high temperature to sinter the stock particles with each other, crushing again and classification, to produce an abrasive of desired particle size and particle size distribution.
The stock material for cerium-based abrasives has been frequently concentrated bastnaesite, a naturally occurring mineral substance obtained by beneficiation of the rare-earth ore by the name of bastnaesite. More recently, the abrasives are produced more frequently from carbonate of cerium-group rare-earth (hereinafter sometimes referred to as the carbonate of rare-earth) or oxide of cerium-group rare-earth (hereinafter sometimes referred to as the oxide of rare-earth). The carbonate of rare-earth is a chemically treated bastnaesite ore or chemically treated relatively-cheap China""s complex ore, which an increased rare-earth metal content, and the oxide of rare-earth is obtained through roasting the carbonate of rare-earth.
The cerium-based abrasive produced from the carbonate of rare-earth as the stock material may sometimes have insufficient grindability. On the other hand, it is pointed out that the cerium-based abrasive produced from the oxide of rare-earth as the stock material tends to leave behind fine scratches on the ground surface, although excellent in grindability.
The present invention is developed to solve the above problems. It is an object of the present invention to provide a cerium-based abrasive having sufficient grindability and, at the same time, little leaving behind fine scratches on a ground surface. It is another object of the present invention to provide a stock material for the above abrasive.
The inventors of the present invention have investigated the processes of producing a cerium-based abrasive for each type of the stock material, noting that the abrasive will have different grinding characteristics by stock material, as discussed above. As a result, they have found that carbonate of rare earth as the stock material has a disadvantage that it is not sintered smoothly during roasting process. Insufficient sintering leads to insufficient grain growth, with the result that it is difficult to prepare a stock material of an adequate particle size and hence to secure sufficient grindability of the resultant abrasive.
On the other hand, it is also found that since an oxide of rare earth sinters smoothly during the roasting process, sintering proceeds unevenly when the oxide of rare earth is used as a stock material, with the result that part of the particles grows excessively. In this case, the abrasive from such a stock material, although excellent in grindability, tends to suffer quality dispersion and leave behind fine scratches on the ground surface, e.g., that of glass.
The inventors of the present invention have found, after having extensively studied while noting difference in properties between the carbonate and oxide of rare earth, that there is a relationship between grindability of a cerium-based abrasive and loss on ignition of the stock material, reaching the present invention.
A first aspect of the invention relates to a stock material for cerium-based abrasives, used for producing cerium-based abrasives, characterized in that it simultaneously contains a carbonate and an oxide of cerium-based rare earth and has a loss on ignition of 0.5 to 25% by weight on a dry basis, when heated at 1000xc2x0 C. for 1 hour.
Loss on ignition (hereinafter also referred to as LOI) is a loss of weight of the sample heated at high temperature. A stock material for cerium-based abrasives (hereinafter referred to as merely stock material) having a higher LOI means that the final product loses more weight from the stock before roasting, to decrease productivity. It is known that LOI of the carbonate of rare earth is high at around 30% and that of the oxide of rare earth is low at around 0.5%. Therefore, LOI can serve as an index which indirectly indicates carbonate/oxide ratio for the present invention. LOI for the present invention is the level determined when the sample is heated at 1000xc2x0 C. for 1 hour, based on the consideration that temperature of 1000xc2x0 C. will give the level applicable as the most stable index, because the carbonate of rare earth is experimentally confirmed to have a stable LOI level when heated at 500xc2x0 C. or higher. The method of determining LOI is specified in JIS-K-0067 (1992, Japanese Standards Association).
The stock material of the present invention for cerium-based abrasives has an LOI of 0.5 to 25% on a dry basis, determined by heating the sample at 1000xc2x0 C. for 1 hour, preferably 1.0 to 25%, more preferably 1.0 to 20%. The stock material having an LOI in the above range for production of cerium-based abrasives secures a higher roasting productivity than the carbonate of rare earth, and, at the same time, prevents uneven sintering, which may occur when the oxide of rare earth is used, thereby allowing the sintering process to proceed more uniformly. As a result, it gives a better cerium-based abrasive both in grindability and grinding precision. More uniform sintering improves classification efficiency, contributing to improved productivity. The stock material thus produced has a lower LOI than the carbonate of rare earth, making the stock lighter by that, and reducing the stock material cost, including transportation cost.
In particular, the abrasive from a stock material having an LOI of 5.0 to 25%, more preferably 5.0 to 20%, has a higher grinding precision and leaves behind less scratches on the ground surface, e.g., that of glass. These advantages make the abrasive suitable for secondary grinding (abrasion for finishing) ofhighly functional glass, e.g., glass substrates for optical disks and magnetic disks. On the other hand, the abrasive from an abrasive atock having an LOI of 0.5 to 5.0%, more preferably 1.0 to 5.0%, is particularly excellent in grindability, and suitable for purposes that require high-speed grindability, e.g., primary grinding of the above-described highly functional glass.
In accordance with the Preferred Embodiments of the present invention, cerium oxide preferably accounts for 40% by weight or more of TREO, more preferably 50% by weight or more. The abrasive comprising cerium oxide in the above range can realize better grindability.
Next, one of the preferable methods of producing the stock material as the first aspect of the invention for abrasives is described.
A second aspect of the invention relates to a method of producing a stock material comprising, as main ingredients, a mixture of carbonate and oxide of cerium-based rare earth, wherein the carbonate/oxide weight ratio is set to give LOI in a range of 0.5 to 25% on a dry basis, determined by heating the mixture at 1000xc2x0 C. for 1 hour.
For this method, the carbonate and oxide of rare earth are first prepared. These two types of crude materials are mixed with each other in a ratio to give an LOI of 0.5 to 25% on a dry basis, determined by heating the mixture at 1000xc2x0 C. for 1 hour, preferably 1.0 to 25%, more preferably 1.0 to 20%, to prepare the stock material mixture for abrasives. More concretely, the carbonate and oxide of rare earth are weighed to have the following weight ratio for the stock material mixture:
LE=LAxc3x97WA/(WA+WB)+LBxc3x97WB/(WA+WB)xe2x80x83xe2x80x83(1)
wherein,
LE: target loss on ignition (0.5 to 25%)
LA: loss on ignition (LOI) of the oxide of rare earth as one of the crude materials
LB: loss on ignition (LOI) of the carbonate of rare earth as one of the crude materials
WA: weight, on a dry basis, of the oxide of rare earth as one of the crude materials
WB: weight, on a dry basis, of the carbonate of rare earth as one of the crude materials
It is preferable, when the above formula is used, to have LOI levels of the carbonate and oxide of rare earth beforehand, because the desired LOI level of the stock material can be finely controlled with these values. When LOI levels of the carbonate and oxide of rare earth are not known beforehand, the level may be determined by the method described later (First Embodiment). The standard LOI levels of the carbonate and oxide of rare earth may be also used. In this case, the prepared carbonate and oxide of rare earth are used as the crude materials without having strict LOI levels. Examples of the standard LOI levels are approximately 30% and 0.5% for the carbonate and oxide rare earth, respectively, although not limited thereto.
One of the advantages of this method is that the stock material mixture of desired LOI level can be prepared by a simple procedure of mixing the oxide and carbonate rare earth with each other in a specific weight ratio, when these materials are available.
It is essential, for a cerium-based abrasive securely having sufficient grindability, to sinter the stock particles into the abrasive particles of an adequate size by roasting, for which the carbonate or oxide of rare earth as the crude material is normally roasted at a relatively high temperature of around 1000xc2x0 C. This is based on the empirical knowledge that the stock carbonate or oxide particles may not be sufficiently sintered unless roasting temperature is increased to the above level.
Increasing roasting temperature is accompanied by a disadvantage of causing abnormal particle growth, although bringing about an advantage of accelerated sintering. The abnormal growth may produce the coarse particles, which may sometimes enter the final product, i.e., abrasive. Content of these coarse particles must be reduced as far as possible, because they may scratch the surface the abrasive tries to grind. They are usually removed by the classification process, subsequent to the roasting process, which adjusts the particle size. The severe classification conditions to remove the coarse particles will decrease productivity of the abrasive, pushing up its production cost.
It is therefore preferable to decrease roasting temperature as far as possible, in order to control the abnormal particle growth during the roasting process, thereby controlling contamination of the abrasive with the coarse particles while securing productivity.
The inventors of the present invention have extensively studied to develop a method of producing an stock material which can sinter the stock particle at a relatively low roasting temperature for production of the abrasives while causing no abnormal growth of the particles, and the stock material produced by the above method and cerium-based abrasive therefrom capable of giving high-quality ground surfaces. They have studied, during the above process, the mechanisms involved in the roasting of the carbonate and oxide of rare earth. They are now sustaining the following phenomena as the reasons why the carbonate and oxide of rare earth need roasting at high temperature:
FIG. 1 illustrates the phenomena involved in roasting of the carbonate of rare earth. The as-received carbonate of rare earth as the stock material is composed of agglomerates with the coarse carbonate of rare earth particles bound to each other. The process of producing an abrasive from the carbonate of rare earth starts with crushing of the stock material. The agglomerated carbonate of rare earth is composed of the particles strongly bound to each other, and is frequently subjected to wet crushing in which the slurried stock material is crushed. The slurry is sufficiently viscous for the cohesive force which bounds the carbonate particles to each other, deteriorating crushing efficiency and making it difficult to completely crush them into the fine particles. As a result, the coarse particles partly remain in the crushed fine particles of the carbonate of rare earth.
The crushed stock material is fluorination-treated, wherein part of the carbonate component in the carbonate of the rare earth is exchanged with fluorine. The coarse particles are disintegrated while the carbonate is partly fluorinated. However, the coarse particles cannot be completely disintegrated, because of quantity of fluorine for the fluorination is limited from the fluorine content of the final product.
The partly fluorinated carbonate of rare earth is then subjected to roasting, wherein most of the carbonate component in the stock material is released as CO2. This leaves low-density, porous, shell-like particles of the carbonate of rare earth. Sintering of these shell-like particles is so low and needs high temperature to proceed. In particular, the carbonate of rare earth contains a high proportion of coarse particles, as described earlier, which become very slow-sintering shell-like particles during the roasting process. Therefore, high roasting temperature is needed for the carbonate of rare earth as the stock material for abrasives.
FIG. 2 illustrates the sintering mechanism involved in roasting of the oxide of rare earth. The oxide of rare earth is produced by calcination of the carbonate of rare earth, as described earlier, and the carbonate as the stock material for the oxide contains coarse particles, as shown in FIG. 1. Calcination of the carbonate releases the carbonate component to form the shell-like particles, which are fragile and disintegrated by the impacts to which they are exposed during the roasting process with the result that the carbonate particles, fine to some extent, are formed. These fine carbonate particles are oxidized by the subsequent heating into the oxide particles.
However, the oxide particles are sintered and agglomerated to each other during the high-temperature calcination process. They are bound to each other strongly, some retaining their shapes even when subjected to the crushing process. The fluorination treatment cannot completely fluorinate these particles, leaving the oxide at the center.
The uneven fluorination should have adverse effects on the sintering process while the particles are being roasted. Namely, these unevenly fluorinated agglomerates are disintegrated under heating and impacts during the roasting process, leaving a mixture of the sufficiently-fluorinated oxide particles and the others not fluorinated or fluorinated only to have an insufficient fluorine content. The former is sintered fast, whereas the latter cannot be sintered fast unless roasting temperature is increased to a fairly high level. Therefore, high roasting temperature is needed for the oxide of rare earth as the stock material for abrasives.
The inventors of the present invention presumed, taking into consideration the above-described hypothetical sintering mechanisms of the carbonate and oxide rare earth, that the above problems would be solved by calcinating the carbonate particles prior to crushing, as is the case with the oxide particles, in order to partly convert the carbonate into the oxide, as the stock-producing method capable of effecting uniform fluorination. The partial calcination process is illustrated in FIG. 3.
The partial calcination process treats the carbonate of rare earth in a manner similar to the process for producing the oxide of rare earth, and changes the carbonate similarly during the initial stage. In other words, the carbonate component is released as CO2, leaving behind the shell-like particles, which are disintegrated into the finer carbonate particles. These particles are oxidized, to have oxide content of the particles increasing with heating time. The partial calcination process for the present invention stops calcination before the carbonate is completely converted into the oxide, leaving a mixture of the carbonate and oxide for the stock material.
The mixed rare earth particles produced by the partial calcination process are subsequently subjected to the crushing and fluorination processes to disintegrate the residual shell-like particles and further crush the particles. The fluorination process, free of the agglomerated particles which are present in the case of the oxide treatment, can be effected uniformly. As a result, the particles are sintered at relatively low temperature during the calcination process, because of lack of the causes for retarding the sintering, e.g., shell-like particles and insufficiently fluorinated particles.
In this way, the partial calcination the inventors propose can produce a stock material while avoiding the problems that the carbonate and oxide rare earth are difficult to sinter unless treated at high temperature. These findings have led to development of the following method, the third invention.
A third aspect of the invention relates to a method of producing a stock material for cerium-based abrasives comprising, as main ingredients, a mixture of carbonate and oxide of cerium-based rare earth, wherein the carbonate of cerium-based rare earth is partly converted into the oxide by calcination under conditions of temperature and time adjusted in such a way to give a stock material mixture LOI of 0.5 to 25% on a dry basis, determined by heating the mixture at 1000xc2x0 C. for 1 hour.
For this method, the carbonate of rare earth is first prepared. The crude material of the carbonate of rare earth is partly converted into the oxide by calcination, to produce the stock material mixture of the carbonate and oxide for abrasives. The calcination decomposes the carbonate into the oxide. Calcination temperature and time are adjusted in such a way to give stock material mixture LOI of 0.5 to 25% on a dry basis, determined by heating the mixture at 1000xc2x0 C. for 1 hour, preferably 1.0 to 25%, more preferably 1.0 to 20%. Calcination temperature and time are not limited, and may be adequately set to give a desired LOI. However, calcination temperature is preferably 150 to 850xc2x0 C., more preferably 400 to 850xc2x0 C., and calcination time is preferably 60 hours or less, more preferably 6 minutes to 48 hours, still more preferably 10 minutes to 24 hours.
This method needs only one type of the crude material, the carbonate of rare earth, which is one of its advantages. It is not necessary to prepare two or more types of the crude materials, which makes this method very economical and high in productivity. It can adjust LOI of the stock material at a desired level by very simple procedure of controlling temperature and time for calcination.
The stock material of the first aspect of the invention, or the one produced by the method of the second or third aspect of the invention gives the cerium-based abrasive excellent in grindability and leaving behind little scratches on the ground surface. More concretely, the stock materials prepared are subjected to the normal process for producing abrasives, comprising slurrying and/or wet crushing, treatment with a mineral acid (as required), fluorination (also as required), filtration, drying, and roasting. It is preferable that the fluorination, if adopted, is effected with the aid of ammonium fluoride and/or hydrofluoric acid.
It is essential for the partial calcination the inventors have proposed how the carbonate of rare earth is oxidized to a desired extent. When excessively heated during the calcination process, the carbonate of rare earth will be completely converted into the oxide, causing uneven fluorination as discussed earlier. When insufficiently heated, on the other hand, the shell-like particles are not sufficiently disintegrated. In either case, the stock material will have insufficient sinterability. The inventors of the present invention have extensively studied to find out the differential calcination conditions for production of stock material, reaching the fourth invention.
A fourth aspect of the invention relates to a method of producing a stock material for cerium-based abrasives comprising, as the main ingredients, a mixture of carbonate and oxide of cerium-based rare earth, wherein the carbonate is calcined at 400 to 850xc2x0 C. to be partly converted into the oxide.
The reason for setting calcination temperature at 400 to 850xc2x0 C. is that the carbonate of rare earth can release the carbonate component to an adequate extent. At higher than 850xc2x0 C., the carbonate is quickly oxidized and completely converted into the oxide. At lower than 400xc2x0 C., on the other hand, both release of the carbonate component and disintegration of the coarse particles are insufficient. Calcination time is preferably 0.1 to 48 hours, as described in claim 12, when it is effected at a temperature in the above range, to similarly release the carbonate component from the carbonate to an adequate extent.
The stock material mixture produced by this method are directly applicable to the stock materials for conventional production process for producing cerium-based abrasives, and the crushing efficiently removes the coarse particles and fluorination uniformly fluorinates the stock particles, to decrease temperature for the roasting process.
The more preferable stock material contains the one for cerium-based abrasives and has an LOI of 1.0 to 20% on a dry basis, determined by heating the sample at 1000xc2x0 C. for 1 hour, viewed from sinterability, convenience for transportation and productivity of the abrasive as the final product, as described in claim 14.
The reason for setting LOI at 1.0 to 20% is that the stock material sufficiently and uniformly sintered at relatively low temperature during the roasting process when it has an LOI in the above range, according to the test results obtained by the inventors, to give the abrasive having relatively high grindability and leaving behind little scratches on the ground surface. The stock material can have an LOI of 1.0 to 20% by heating the carbonate of rare earth in such a way to adjust the carbonate/oxide ratio for the mixture having a desired LOI during the calcination process effected under the above-described temperature and time conditions. LOI may be also adjusted at a desired level by incorporating the stock material mixture produced by this method with the carbonate or oxide of rare earth.
The stock material related to the present invention can be sintered at a sufficient rate even at relatively low temperature, as discussed above. The invention of claim 15 of the present invention relates to the method of producing a cerium-based abrasive, comprising crushing and fluorinating the above stock material, and roasting the fluorinated stock material at 700 to 1000xc2x0 C. Roasting at such a relatively low temperature level can control abnormal growth of the particles for the cerium-based abrasive which can give a high-quality surface free of scratches.
This method of producing a cerium-based abrasive involves fluorination prior to roasting, which is preferably effected in the presence of hydrofluoric acid or ammonium fluoride. The latter is more preferable, because it allows the fluorination process to proceed at a mild rate to distribute fluorine more uniformly in the stock material, and hence the roasting process to be effected at a lower temperature level.